Bug 1248152 - systemd detects backlight (and fails to manipulate it) when no backlight exists
systemd detects backlight (and fails to manipulate it) when no backlight exists
Product: Fedora
Classification: Fedora
Component: systemd (Show other bugs)
Unspecified Unspecified
unspecified Severity unspecified
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Assigned To: systemd-maint
Fedora Extras Quality Assurance
Depends On:
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Reported: 2015-07-29 13:51 EDT by Jason Tibbitts
Modified: 2016-07-19 13:15 EDT (History)
8 users (show)

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Doc Type: Bug Fix
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Last Closed: 2016-07-19 13:15:25 EDT
Type: Bug
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dmesg (55.25 KB, text/plain)
2015-07-29 13:51 EDT, Jason Tibbitts
no flags Details

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Description Jason Tibbitts 2015-07-29 13:51:05 EDT
Created attachment 1057376 [details]

I'm having an issue with a large number of desktops where systemctl status will show the state as "degraded" because systemd-backlight@backlight:acpi_video0.service fails at boot time.  systemctl status shows:

Jul 28 12:53:00 ld61.e.math.uh.edu systemd[1]: Starting Load/Save Screen Backlight Brightness of backlight:acpi_video0...
Jul 28 12:53:00 ld61.e.math.uh.edu systemd[1]: systemd-backlight@backlight:acpi_video0.service: main process exited, code=exited, status=1/FAILURE
Jul 28 12:53:00 ld61.e.math.uh.edu systemd[1]: Failed to start Load/Save Screen Backlight Brightness of backlight:acpi_video0.
Jul 28 12:53:00 ld61.e.math.uh.edu systemd[1]: Unit systemd-backlight@backlight:acpi_video0.service entered failed state.
Jul 28 12:53:00 ld61.e.math.uh.edu systemd[1]: systemd-backlight@backlight:acpi_video0.service failed.

Now, /sys/class/backlight is empty, so I'm not sure what method systemd is using to detect a backlight.  I've attached dmesg output.

Hans suggested that there might be a race where a backlight exists momentarily only to be removed after systemd has already failed to manipulate it, and from looking at the kernel log it does appear to be a possibility.  He asked me to file this ticket and CC him.
Comment 1 Hans de Goede 2015-07-29 14:02:24 EDT
Hi All,

So what I believe is happening here is that the involved Asus mainboards suffer from a known Asus issue where Asus exports an acpi-video backlight interface on desktop/server mainboards.

The kernel has special Asus specific code to deal with this in drivers/platform/x86/asus_wmi.c, which
unregisters the acpi-video backlight if it has already registered and sets a flag telling the acpi-video code to not register it later on.

So depending on module load ordering the acpi-video0 backlight sysfs interface may briefly appear during module loading. Note that this is not unique to Asus. many of the vendor specific drivers/platform/x86/vendor-foo.c drivers do the same. It was decided long ago to have vendor specific handling in vendor specific drivers even if it involves removing the acpi-backlight interface because it is know to be broken for certain boards of that vendor.

I realize that this far from ideal from a systemd pov, but there is very little we can do about this in the kernel (if only firmware did not suck ...), so it would be good if a workaround could be added to systemd to not consider it a failure when an acpi_video# backlight interface disappears underneath it.


Comment 2 Zbigniew Jędrzejewski-Szmek 2015-07-29 14:09:27 EDT
We should be able to deal with hardware going away at any time (for example being unplugged). So we should ignore missing devices in general.
Comment 3 Fedora End Of Life 2016-07-19 13:15:25 EDT
Fedora 22 changed to end-of-life (EOL) status on 2016-07-19. Fedora 22 is
no longer maintained, which means that it will not receive any further
security or bug fix updates. As a result we are closing this bug.

If you can reproduce this bug against a currently maintained version of
Fedora please feel free to reopen this bug against that version. If you
are unable to reopen this bug, please file a new report against the
current release. If you experience problems, please add a comment to this

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